Philipp Meinecke sent me a link to http://tolweb.org/images/Helicopsychidae/14641. Always good to have a look on the other side of the fence (i.c. entomology).
This text and figures by Karl Kjer explain:
The snail-case caddisflies of the family Helicopsychidae were first recognized as the subfamily Helicopsychinae of Sericostomatidae by Ulmer (1906) and were retained there by a number of European workers well into the 1950s, most notably Ulmer himself (Ulmer 1955). Ross (1944) and other American workers considered the group a distinct family, reflecting its current status. As presently constituted, the family contains only 2 genera, the cosmopolitan Helicopsychevon Siebold with about 250 species, and the New Zealand endemic genus Rakiura McFarlane, with a single species, R. vernale McFarlane. Several previously recognized genera, including Cochliopsyche Müller (Neotropical), Petrotrichia Ulmer (Afrotropical, including Madagascar and the Seychelles, but absent from southern Africa), and Saetotrichia Brauer (Australia, New Zealand, New Caledonia), were relegated as subgenera of Helicopsyche by Johanson (1998). In the same paper, Johanson described 2 additional subgenera of Helicopsyche: Feropsyche (Nearctic, Neotropical) and Galeopsyche(Korea, Vietnam). The nominotypical subgenus occurs in the Palearctic and Oriental regions. As a whole the family is poorly represented in the Northern Hemisphere, but reaches its greatest diversity in the tropics of the Old and New Worlds (Johanson 1997); the Neotropics alone hosts about 100 species. Taken from Holzenthal et al. (2007).
Larvae of the genus are the familiar and remarkable snail-case builders. These helical, sand grain cases are so similar to snails that early workers described these insects as molluscs. Lea (1834) went so far as to say of Valvata arenifera(=Helicopsyche borealis), “It has the singular property of strengthening its whirls by the agglutination of particles of sand, and by which it is entirely covered.” While all helical, there is great diversity in the height of cases, the number and openness of the whorls, the size of mineral material, and the amount of silk incorporated. All helicopsychid larvae appear to feed as scrapers on periphyton and other organic matter on the exposed surfaces of rocks. They are found in slow flowing lowland streams as well as springs, small fast-flowing streams, and the wave-washed shores of lakes in temperate regions; they also occur in the hyporheic zone (Williams et al. 1983) and in thermal springs (Resh et al. 1984). The biology of the North American species, H. borealis (Hagen) is well known (Vaughn 1985a, b, 1987). Taken from Holzenthal et al. (2007).
Since Morse’s (1997) review of phylogenetic studies within the Trichoptera, Johanson has undertaken significant analyses of evolutionary relationships within Helicopsyche (Johanson 1998, 2001, 2002, Johanson & Willassen 1997).Taken from Holzenthal et al. (2007).
Holzenthal R.W., Blahnik, R.J., Prather, A.L., and Kjer K.M. 2007. Order Trichoptera Kirby 1813 (Insecta), Caddisflies. In: Zhang, Z.-Q., and Shear, W.A. (Eds). 2007 Linneaus Tercentenary: Progress in Invertebrate Taxonomy. Zootaxa 1668:639-698
Johanson, K.A. (1997) Zoogeography and diversity of the snail case caddisflies (Trichoptera: Helicopsychidae). In: Holzenthal, R.W. & Flint, O.S., Jr. (Eds.) Proceedings of the 8th International Symposium on Trichoptera. Ohio Biological Survey, Columbus, Ohio, pp. 205–212.
Johanson, K.A. (1998) Phylogenetic and biogeographic analysis of the family Helicopsychidae (Insecta: Trichoptera). Entomologica Scandinavica, Supplement, 53, 1–172.
Johanson, K.A. (2001) Phylogenetic and biogeographical analysis of the New Zealand Helicopsyche von Seibold (Trichoptera: Helicopsychidae). Insect Systematics and Evolution, 32, 107–120.
Johanson, K.A. (2002) A new primitive Helicopsyche from Madagascar (Trichoptera: Helicopsychidae), with phylogenetic analysis of Afrotropical species. Tijdschrift voor Entomologie, 145.
Johanson, K.A. & Willassen, E. (1997) Are the African species of Helicopsyche von Siebold 1856 (Insecta Trichoptera Helicopsychidae) monophyletic? Tropical Zoology, 10, 117–128.
Lea, I. (1834) Observations on the Naiades, and descriptions of new species of that and other families. Transactions of the American Philosophical Society, 4, 63–121.
Morse, J.C. (1997) Phylogeny of Trichoptera. Annual Review of Entomology, 42, 427–450.
Resh, V.H., Lamberti, G.A. & Wood, J.R. (1984) Biological studies of Helicopsyche borealis (Hagen) in a coastal California stream. In: Morse, J.C. (Ed.) Proceedings of the 4th International Symposium on Trichoptera. Dr. W. Junk, The Hague, pp. 315–319.
Ross, H.H. (1944) The caddisflies or Trichoptera of Illinois. Bulletin of the Illinois Natural History Survey, 23, 1–326.
Ulmer, G. (1906) Neuer beitrag zur kenntnis außereuropäischer Trichopteren. Notes from the Leyden Museum, 28, 1–116.
Ulmer, G. (1955) Köcherfliegen (Trichopteren) von den Sunda-Inseln. Teil II. Larven und Puppen der Integripalpia.Archiv für Hydrobiologie, Supplement, 21, 408–608.
Vaughn, C.C. (1985a) Evolutionary ecology of case architecture in the snailcase caddisfly, Helicopsyche borealis. Freshwater Invertebrate Biology, 54, 178–186.
Vaughn, C.C. (1985b) Life history of Helicopsyche borealis (Hagen) (Trichoptera: Helicopsychidae) in Oklahoma. American Midland Naturalist, 113, 76–83.
Vaughn, C.C. (1987) Substratum preference of the caddisfly Helicopsyche borealis (Hagen) (Trichoptera: Helicopsychidae). Hydrobiologia, 154, 201–205.
Williams, D.D., Read, A.T. & Moore, K.A. (1983) The biology and zoogeography of Helicopsyche borealis (Trichoptera: Helicopsychidae): a Nearctic representative of a tropical genus. Canadian Journal of Zoology, 61, 2288–2299.
An interesting and potential case of mimicry. Or just a possibility for confusion. Some of these caddisfly shells have already been spotted in a malacological collection, so curators may want to check their holdings…